FIG. 8 shows an outline of a surface structure of a conventional magneto-optical recording medium. The magneto-optical recording medium comprises a multi-layered film 26 composed of at least one layer of a magnetic recording film and a reflective film or the like formed on a transparent substrate 10 via an insulative film. The substrate 10 includes grooves 22 and prepits 24 which are previously formed in a direction along the track by means of injection molding. Lands (projections) 28 are defined between the grooves 22. The land 28 includes a data-recording area for recording information thereon, and a header area in which, for example, an address signal is recorded in a form of prepit 24. The track position control (tracking) is performed as follows when data are recorded and reproduced. That is, for example, the push-pull system is adopted, in which the groove 22 is irradiated with a light spot to detect diffracted light beams subjected to diffraction from the groove 22 in a divided manner in right and left directions perpendicular to the track respectively, and the track position is controlled so that the diffracted light beams have an equal intensity. The tracking must be correctly performed in order to write data at a desired position. For this purpose, it is necessary to highly accurately process the grooves 22 and the lands 28.
The substrate, which has the grooves 22 and the prepits 24 as described above, is usually formed in accordance with the following procedure. At first, a flat base disk made of glass, to which a photoresist is applied to give a predetermined thickness, is prepared. A master disk exposure apparatus is used to expose portions corresponding to the grooves and the prepits with a laser spot while rotating the glass base disk at a constant speed. The exposed base disk is subjected to a development process to remove the exposed portions so that a master disk is produced. A stamper is formed on the basis of the master disk. Further, the stamper is installed in a molding mold to perform injection molding for a plastic material with the mold. Thus, the substrate for the optical recording medium having the predetermined grooves and the prepits is reproduced.
An optical recording medium, which makes it possible to perform high density recording, is demanded as the amount of information to be processed is increased. In the case of the optical recording medium having the structure as shown in FIG. 8, the track pitch should be made narrow in order to achieve a high density. For this reason, when the groove portion is subjected to exposure by using the master disk exposure apparatus, an optical blur occurs at the land 28 adjacent to the groove 22. That is, the laser beam for exposure usually has an intensity distribution which follows the Gaussian distribution curve. The diameter of the laser spot for exposure is substantially determined by the half value width. Therefore, the outside of the portion corresponding to the groove (edge of the land) is also subjected to exposure. Accordingly, the portion corresponding to the land tends to cause surface roughness during the development process. The width and the height of the land are decreased due to the surface roughness, resulting in decrease in S/N ratio of the data signal.
An optical recording medium, for example, a so-called wide-width groove type magneto-optical disk is known concerning a structure of the optical recording medium for dissolving the problem described above, in which grooves 12 having a wide width are formed as shown in FIG. 1, and data are written in the grooves 12. In the case of the optical recording medium having such a structure, the recording signal is recorded on the groove 12 having the wide width, not on the land on which the optical blur tends to occur. Therefore, no surface roughness occurs on the recording surface. Especially, the bottom surface 13 of the groove 12 corresponds to the surface of the base material of the base disk which has excellent flatness on the master disk. Accordingly, when data are written on this surface, it is possible to avoid deterioration of S/N.
However, in the case of the substrate having the wide-width type groove, it is necessary that the portion 12 corresponding to the groove is exposed with a light beam having a relatively high intensity in order to form the groove having the wide width. As described above, data are written on the recess in the case of the optical recording medium based on the use of the substrate having the wide-width type groove. Therefore, such an optical recording medium is free from any fear of surface roughness on the recording area, but the optical blur occurs on the narrow land 11. Especially, an optical recording medium having a track pitch of not more than 1 .mu.m, in which the track is made narrow in order to realize the high density, is greatly affected by the blur caused on the land. Further, for example, if there is any fluctuation of exposure beam intensity, or if uneven feed is caused in the radial direction during exposure effected by the master disk exposure apparatus, the change in shape of the land tends to occur at such portions. In order to avoid the change in shape due to uneven feed as described above, for example, Japanese Patent Application Laid-Open No. 1-201844 discloses a method for producing a master disk for an optical disk in which first and second laser beams are radiated in the radial direction while being separated from each other by a predetermined spacing distance during master disk exposure so that first and second exposure zones are formed on a resist film with a non-exposed zone being interposed therebetween. When this system is adopted, the change in shape of the land is small with respect to the uneven feed in the radial direction. However, the fluctuation tends to occur concerning the width of the groove on which data are recorded, resulting in decrease in S/N of a reproduced signal obtained from the data-recording area. Further, in the case of the optical recording medium having the wide-width groove, the land to serve as the projection is relatively narrow. For this reason, a gap is formed between the stamper and the resin material, and it is relatively difficult to transfer the land during molding. Therefore, the change in shape of the land tends to occur due to defective transfer. Japanese Patent Application Laid-Open No. 1-201844 refers to nothing about the countermeasure for dissolving the defective transfer. The defective transfer conspicuously occurs as the track pitch becomes narrow.
The following problem arises upon reproduction of a signal recorded on an optical recording medium having the wide-width groove in which the change in shape of the land has occurred as described above. That is, the track position control signal varies due to disturbance of the diffracted light beam from the groove, and it is impossible to perform correct tracking. This problem is more serious in the case of a high density recording medium having a track pitch of not more than 1 .mu.m.
Japanese Patent Application Laid-Open No. 6-338064 discloses an optical disk for recording signals on both of guide grooves and lands. In this case, the depth of the guide groove has an optical length of not less than .lambda./7 and not more than .lambda./5, and the width of the guide groove is not less than 0.34.times.R and not more than R (R represents a beam diameter of an information-recording laser beam on a recording layer). Thus, the optical disk is prevented from deterioration of the reproduction error rate which would be otherwise caused due to the erasing residue during overwrite and the outward extension of recording marks.
Japanese Patent Application Laid-Open No. 63-18535 discloses a substrate for an optical recording medium in which pregrooves and address pits are formed on an identical track to satisfy .lambda./(8n).ltoreq.d.ltoreq..lambda./(4n) and P/2.ltoreq.W&lt;P provided that the wavelength of a recording and reproducing beam is .lambda., the refractive index of a light-transmissive layer is n, the track pitch is P, the phase depth of the pregroove and the address pit is d, and the width is W. This patent document reports that the use of the substrate makes it possible to improve S/N by suppressing variation in width of the land during master disk exposure by using a cutting apparatus.
An object of the present invention is to provide a high density optical recording medium having wide-width grooves which dissolves the conventional problems as described above and which makes it possible to reliably perform tracking.
As a result of repeated experiments to maintain the stability of track position control while maintaining high S/N performance of wide-width grooves in order to solve the problems as described above, the present inventors have succeeded in developing an optical recording medium capable of stable tracking by constructing the optical recording medium such that the height of lands of the optical recording medium having wide-width grooves is within a predetermined range.